Apparatus and method of making a variable stiffness multilayer catheter tubing

ABSTRACT

A catheter body having a variable stiffness along its longitudinal length and a method for manufacturing same is disclosed wherein an inner layer having an uninterrupted length serves as a backbone for segments of coextrusion of, e.g., Pebax or nylon and a tie layer which are then bonded to the backbone to create a multi-stiffness catheter body.

BACKGROUND

This invention generally relates to catheters, and particularly tointravascular catheters for use in percutaneous transluminal coronaryangioplasty (PTCA) or for the delivery of stents.

In percutaneous transluminal coronary angioplasty (PTCA) procedures, aguiding catheter is advanced in the patient's vasculature until thedistal tip of the guiding catheter is seated in the ostium of a desiredcoronary artery. A guidewire is first advanced out of the distal end ofthe guiding catheter into the patient's coronary artery until the distalend of the guidewire crosses a lesion to be dilated. A dilatationcatheter, having an inflatable balloon on the distal portion thereof, isadvanced into the patient's coronary anatomy over the previouslyintroduced guidewire until the balloon of the dilatation catheter isproperly positioned across the lesion. Once properly positioned, thedilatation balloon is inflated with inflation fluid one or more times toa predetermined size at relatively high pressures so that the stenosisis compressed against the arterial wall and the wall expanded to open upthe vascular passageway. Generally, the inflated diameter of the balloonis approximately the same diameter as the native diameter of the bodylumen being dilated so as to complete the dilatation but not over expandthe artery wall. After the balloon is finally deflated, blood resumesthrough the dilated artery and the dilatation catheter and the guidewirecan be removed.

In such angioplasty procedures, there may be restenosis of the artery,i.e., reformation of the arterial blockage, which necessitates eitheranother angioplasty procedure, or some other method of repairing orstrengthening the dilated area. To reduce the restenosis rate ofangioplasty alone and to strengthen the dilated area, physicians mayimplant an intravascular prosthesis, generally called a stent, insidethe artery at the site of the lesion. Stents may also be used to repairvessels having an intimal flap or dissection or to generally strengthena weakened section of a vessel or to maintain its patency.

Stents are usually delivered to a desired location within a coronaryartery in a contracted condition on a balloon of a catheter which issimilar in many respects to a balloon angioplasty catheter, and expandedwithin the patient's artery to a larger diameter by expansion of theballoon. The balloon is deflated to remove the catheter and the stentleft in place within the artery at the site of the dilated lesion. Fordetails of stents, see for example, U.S. Pat. No. 5,507,768 (Lau, etal.) and U.S. Pat. No. 5,458,615 (Klemm, et al.), which are incorporatedherein by reference.

An essential step in effectively performing a PTCA procedure is properlypositioning the balloon catheter at a desired location within thecoronary artery. To properly position the balloon at the stenosedregion, the catheter must have good pushability (i.e., ability totransmit force along the length of the catheter), and good trackabilityand flexibility, to be readily advanceable within the tortuous anatomyof the patient's vasculature. Conventional balloon catheters forintravascular procedures, such as angioplasty and stent delivery,frequently have a relatively stiff proximal shaft section to facilitateadvancement of the catheter within the patient's body lumen and arelatively flexible distal shaft section to facilitate passage throughtortuous anatomy such as distal coronary and neurological arterieswithout damage to the vessel wall. These flexibility transitions can beachieved by a number of methods, such as bonding two or more tubingsegments of different flexibility together to form the shaft. However,such transition bonds must be sufficiently strong to withstand thepulling and pushing forces on the shaft during use. At present, however,there are distinct shortcomings associated with the methods ofmanufacture proposed to produce a catheter with this characteristic. Inparticular, current methods do not satisfy the necessary tensilestrength requirements set for such devices.

One proposed method of creating a varying stiffness catheter involvescutting segments of different multi-layer tubular members and joiningthem together end to end, with the outermost layer of the distalsegment(s) having a reduced durometer and/or thickness compared withthat of its adjacent more proximal segment. While this technique wasused to produce samples which were bench tested to prove the merit ofthe technology with regard to deliverability, the joints created by themating of the segments were not sufficiently robust to meet producttensile strength and other reliability requirements. The reason for thisis that most catheter tubings are multi-layers, such as tri-layerextrusions. These three-layer configurations have an innermost layerthat is particularly difficult to join end-to-end, because the innermostlayer is typically constructed of high density polyethylene (HDPE),which is not melt-bond compatible with a nylon or Pebax outermost layer.Attempts to either butt-join or lap-join tri-layer inner member segmentshave been unsuccessful because all abutting or overlapping layers didnot bond reliably to one another.

Past approaches to improve joint reliability and overallmanufacturability have all suffered several common drawbacks: the tubingneeded to be heated along its entire length to bond the various piecestogether; and an entire length of shrink tubing covering the length ofthe tubing must be used to bond the layers and then discarded. For rapidexchange balloon catheters and other catheters, where the variablestiffness tubing spans approximately 25 cm within the finished device,the extended shrink tubing amounts to a considerable overall costincrease (multiple extrusions, shrink tubing, more direct labor requiredfor assembly) relative to a conventional tri-layer extrusion. For overthe wire balloon catheters, in which the variable stiffness tubing canbe approximately five times longer, the cost becomes essentiallyprohibitive.

SUMMARY OF THE INVENTION

This present invention is an apparatus and method for producing amulti-layer catheter tubing whose outermost layer is comprised ofpolymers of different stiffness along its length and the innermost layeris an uninterrupted lubricious layer. This invention enables theassembly of various outer layer segments, including tungsten-filledpolymer segments serving as balloon markers, onto the uninterruptedinnermost layer without having to process the entire length of thetubing with progressive heating and shrink tubing. Rather, only shortdistal regions require heat and shrink tubing. This is particularlysignificant for over the wire catheters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevated, perspective view, partially in cut-away, of acatheter of the kind constructed by the present invention;

FIG. 2 is a cross-sectional view of the catheter of FIG. 1 taken alonglines 2-2;

FIG. 3 is a cross-sectional view of the catheter of FIG. 1 taken alonglines 3-3;

FIG. 4 is an enlarged, cross-sectional view of a portion of the body ofthe tubing used to construct the catheter of FIG. 1;

FIG. 5 is an enlarged, perspective view of the distal end of the tubingof FIG. 4;

FIG. 6 is a perspective view of the distal end of the tubing as theouter layer is peeled off;

FIG. 7 is an enlarged, perspective view of a new tubing segment placedover the tubing of FIG. 6;

FIG. 8 is an enlarged, perspective view of the new tubing segment ofFIG. 7 with shrink tubing placed over it;

FIG. 9 is an enlarged, side view of flaps in a proximal layer beingoverlaid over the new tubing segment of FIG. 7; and

FIG. 10 is an enlarged, side view of the proximal inner member withoverlapping flaps cut at an angle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a balloon catheter of the type that can benefit fromthe present invention. The catheter 10 of the invention generallycomprises an elongated catheter shaft 11 having a proximal section, 12 adistal section 13, an inflatable balloon 14 formed of a blend ofpolymeric materials on the distal section 13 of the catheter shaft 11,and an adapter 17 mounted on the proximal section 12 of shaft 11. InFIG. 1, the distal portion of the catheter 10 is illustrated within apatient's body lumen 18, prior to expansion of the balloon 14.

In the embodiment illustrated in FIG. 1, the catheter shaft 11 has anouter tubular member 19 and an inner tubular member 20 disposed withinthe outer tubular member defining, with the outer tubular member, aninflation lumen 21. Inflation lumen 21 is in fluid communication withthe interior chamber 15 of the inflatable balloon 14. The inner tubularmember 20 has an inner lumen 22 extending therein which is configured toslidably receive a guidewire 23 suitable for advancement through apatient's coronary arteries. The distal extremity 31 of the inflatableballoon 14 is sealingly secured to the distal extremity of the innertubular member 20 and the proximal extremity 32 of the balloon 14 issealingly secured to the distal extremity 13 of the outer tubular member19.

FIGS. 2 and 3 show transverse cross sections of the catheter shaft 11and balloon 14, respectively, illustrating the guidewire receiving lumen22 of the guidewire's inner tubular member 20 and inflation lumen 21leading to the balloon interior 15. The balloon 14 can be inflated byradiopaque fluid introduced at the port in the side arm 24 intoinflation lumen 21 contained in the catheter shaft 11, or by othermeans, such as from a passageway formed between the outside of thecatheter shaft 11 and the member forming the balloon, depending on theparticular design of the catheter. The details and mechanics of ballooninflation vary according to the specific design of the catheter, and arewell known in the art.

FIGS. 4-10 illustrate the various steps of constructing the variablestiffness catheter of the present invention. According to the presentinvention, a catheter body is formed as a dual-layer hollow extrusionwith a lubricious inner layer 100 of HDPE or ultra high molecular weightpolyethylene (UHMWPE) and an outer layer 105 of nylon or Pebax, omittingthe usual Primacor “tie layer” that binds the inner layer 100 to theouter layer 105. This extrusion serves as the “backbone” for the finalinner member's entire length. As explained below, other dual-layerhollow extrusions having an inner layer of Primacor and an outer layerof nylon or Pebax will be added to provide varying bending stiffness tothe distal end. Further, a tungsten-filled polymer (nylon or Pebax)hollow extrusion, with or without an inner layer of Primacor, may beutilized to provide radio-opacity at discrete locations to serve asvisual markers.

Assembly begins with the following steps to remove a distal section ofthe “backbone” extrusion's outer Pebax or nylon layer:

Step 1: At an appropriate distance from the distal end, the outer layerof the HDPE/nylon or HDPE/Pebax “backbone” extrusion iscircumferentially scored 110 using a cutting instrument such as a razorblade or the like to create a break point of the outer layer 105 only(FIG. 4). Care is called for to control the scoring blade in order toprotect the inner layer 100.

Step 2: A longitudinal slit 120 is made at the distal end of theextrusion so as to bisect the tubing over a length of several mm or moreusing a cutting knife such as a razor blade or equivalent, creating twosemi-circular halves at the distal end (FIG. 5).

Step 3: To separate the outer layer 105 from the inner layer 100, bothhalves of the bisected end are folded back, and a grasping tool such astweezers or the like is used to grasp the outer layer 105 and pull itaway from the inner layer 100 of each half. The outer layer 105 of eachhalf is then peeled away from their respective inner layer to thusseparate and remove the outer layer until the score mark 110 is reached,whereupon the outer layer halves 105 tear away from the “backbone”extrusion. The result is a stepped transition 140 between the exposedHDPE inner layer 100 and the intact proximal remainder of theextrusion's outer layer 105 (FIGS. 6, 7).

Although other techniques may be used to achieve the same objective, thesteps above describe a simply way to remove a defined length of the toplayer. Note that this objective would be difficult if not impossible toachieve if the “backbone” was a conventional tri-layer extrusion, due tothe tenacious adhesive bond provided by the Primacor middle “tie-layer,”and attempts to do so using the steps above have proven unsuccessful.

Next, segments 150 of appropriate length are cut from Primacor-linednylon and/or Pebax extrusions, and optionally segments of atungsten-filled polymer extrusion (with or without a Primacor lining)when a visual marker is desired, and the segments 150 are slid over theexposed HDPE inner layer 100 of the “backbone” extrusion (see arrow 170of FIG. 7). All segments 150 are butted together and a suitable lengthof shrink tubing 175 is placed over the region (See FIG. 8). Afluoropolymer shrink tube material, such as FEP, is preferable due tothe non-stick nature. This region only is then progressively heated tomelt bond the various segments 150 together and allow the Primacor toadhere or “tie” the segments to the underlying HDPE layer. Afterwards,the shrink tubing 175 and mandrel 130 are removed to leave the finishedcatheter body.

The resultant composite tubing is a variable-durometer inner member,with or without integral tungsten-filled balloon markers, whose innerHDPE layer is uninterrupted. The distal end is effectively a tri-layerand can be processed like any conventional tri-layer inner member withregard to balloon sealing, tip attachment, marker band swaging or fusing(if needed), etc. To make the transition from one durometer section toanother less abrupt, the outer layer 155 of the proximal dual-layerextrusion may be left unscored prior to peeling, so the peeled stripscan be trimmed with flaps 165 remaining. As illustrated in FIG. 9, theseflaps can be made to overlap the adjacent segment 150 before heatingwith shrink tubing, so the overlapped region is comprised of both,albeit thinned, outer layers. The flaps 165 a may be angle-cut to“feather” the stiffness transition, as shown in FIG. 10. Alternatively,the flaps may be purposely trimmed to differing lengths in order tofurther broaden the transition region.

In yet another embodiment, the proximal dual-layer extrusion is trimmedas shown in FIG. 7 and the adjacent segment is slit and made to overlapthe remaining outer layer on the dual-layer extrusion before heatingwith the shrink tubing.

In all instances the proximal end is a dual-layer extrusion whose layersare mechanically bonded by virtue of their intimate proximity and theinherent surface roughness at their interface. The resultingvariable-durometer inner member may be hot die necked, including thedual-layer proximal section, using the same methods as for conventionaltri-layer extrusions. Although adding cost, hot die necking could beused to provide additional changes in stiffness, improved control offinal dimensions, or increased tensile load carrying capability to theinner member. At equal final dimensions, a hot die necked inner memberwill typically have a greater tensile break load then one that has notbeen necked.

The “backbone” extrusion's outer layer 105 can be any durometer polymer,as required by the application, and its inner layer 100 can be anyextrudable lubricious material. However, preferably the layer materialsshould not adhere well to each other during extrusion, because peelingoff the outer layer 105 at the distal end would be more difficult.

The “backbone” extrusion may be E-beam irradiated, particularly if itsinner layer is HDPE (or UHMWPE), as this promotes cross-linking and thusprevents undesirable material flow of the inner layer during subsequentmelt bonding operations.

The added outer layer segments 150 can be any durometer polymer, as theapplication requires, but it is preferred that they contain an innerlayer of a “tie layer” material like Primacor in order to promote securebonding to the “backbone” extrusion's inner layer 100. The heat neededfor such bonding is preferably achieved by equipment that provideslocalized and controllable heat with the ability to traverse or rotate,and the required radial pressure is preferably provided by shrink tubingwhich does not adhere well to the underlying materials. Although itwould be possible to simply heat the assembly in an oven, this is lessdesirable because of a greater tendency to trap air beneath the shrinktubing 175 leading to surface irregularities.

This invention is also applicable to inner members whose inner layer 100is a fluoropolymer such as PTFE. In one alternate embodiment, the innerlayer 100 is a single-layer extrusion that is subsequently etched (e.g.,sodium naphthalene or “Tetra Etch”) to promote bondability of its outersurface. An outer layer 105 is then extruded onto the fluoropolymertubing in a semi-continuous (reel to reel) manner, with the extrusionparameters selected to prevent melt bonding of the two layers. Thus, theouter layer 105 can be subsequently peeled away at one end to make roomfor the installation of various durometers of outer jacket segments 150and tungsten-filled polymer markers. In this embodiment, the addedsegments 150 do not require an inner “tie layer’ because they can bemelt bonded directly to the etched fluoropolymer surface, again usingheat and shrink tubing.

What is claimed:
 1. A method for forming a catheter tubing with avariable stiffness, comprising: providing a length of a multi-layerextrusion to serve as a backbone for the catheter tubing, saidmulti-layer extrusion having an outer layer and an inner layer; removinga portion of the outer layer from a distal end of said multi-layerextrusion to an intermediate location to expose the inner layer; slidinga first extrusion segment having a stiffness that differs from that ofthe outer layer of the multi-layer extrusion over the exposed innerlayer until the first extrusion segment abuts the outer layer of themulti-layer extrusion at the intermediate location; placing a length ofshrink tubing over the abutting portion of the first extrusion segmentand the multi-layer extrusion; and heating the shrink tubing to mate theabutting extrusion segment to the multi-layer extrusion.
 2. The methodof forming a catheter tubing with a variable stiffness of claim 1,further comprising sliding a second extrusion segment having a stiffnessthat differs from that of the first extrusion segment over the exposedinner layer of the multi-layer extrusion until it abuts the firstextrusion segment, placing a length of shrink tubing over the abuttingportion of the first and second extrusion segments, and heating theshrink tubing to mate the abutting extrusion segments to the multi-layerextrusion.
 3. The method of forming a catheter tubing with a variablestiffness of claim 2, wherein said second extrusion segment comprises atungsten-filled polymer extrusion.
 4. The method of forming a cathetertubing with a variable stiffness of claim 1, wherein the inner layer isformed of HDPE.
 5. The method of forming a catheter tubing with avariable stiffness of claim 1, wherein the inner layer is formed ofUHMWPE.
 6. The method of forming a catheter tubing with a variablestiffness of claim 1, wherein the outer layer is comprised of one of thegroup of nylon and Pebax.
 7. The method of forming a catheter tubingwith a variable stiffness of claim 1, wherein the multi-layer extrusiondoes not have a tie layer bonding the inner and outer layers.
 8. Themethod of forming a catheter tubing with a variable stiffness of claim1, wherein the removing of the outer layer begins with scoring themulti-layer extrusion at the intermediate location and then peeling theouter layer away from the inner layer until it breaks off at the scoredlocation.
 9. The method of forming a catheter tubing with a variablestiffness of claim 8, further comprising bisecting a distal end of themulti-layer extrusion prior to peeling away the outer layer.
 10. Themethod of forming a catheter tubing with a variable stiffness of claim1, wherein an outer layer of the first extrusion segment and the outerlayer of the multi-layer extrusion overlap.
 11. The method of forming acatheter tubing with a variable stiffness of claim 10, wherein theoverlapping portions of the first extrusion segment and multi-layerextrusion are angle cut.
 12. The method of forming a catheter tubingwith a variable stiffness of claim 1, further comprising hot die neckingthe catheter tubing.
 13. The method of forming a catheter tubing with avariable stiffness of claim 1, further comprising E-beam irradiating theinner layer of the multi-layer extrusion.
 14. The method of forming acatheter tubing with a variable stiffness of claim 1, wherein the firstextrusion segment includes an inner tie layer to bond with the innerlayer of the multi-layer extrusion.
 15. The method of forming a cathetertubing with a variable stiffness of claim 1, wherein the inner layer ofthe multi-layer extrusion is a fluoropolymer.
 16. The method of forminga catheter tubing with a variable stiffness of claim 15, furthercomprising etching the fluoropolymer layer with sodium naphthalene.